Objective lens driving apparatus

ABSTRACT

A lens holder is provided with objective lenses of different types. Rotating the lens holder causes one of the objective lenses to be placed in the optical path of the laser beam. Near one objective lens, there is provided a reflecting element that reflects part of the laser beam directed to the objective lens. When the objective lens is put in the optical path, the beam from the reflecting element is sensed by a sensor. From the sensing result, it is judged that a specific type of objective lens is in the optical path. Accordingly, there is provided an objective lens driving apparatus that includes at least two objective lenses with different numerical apertures according to optical information recording mediums complying with different standards, is capable of switching between the objective lenses according to the optical information recording medium to be used, and further has a simple structure capable of identifying the type of the objective lens selected by switching.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an objective lens driving apparatus providedon an optical disc apparatus, and more particularly to an objective lensdriving apparatus that not only switches objective lenses with differentnumerical apertures according to the type of a recording medium but alsoidentifies the type of the selected objective lens.

2. Description of the Related Art

With the recent development of optical information recording mediums,including optical discs and magneto-optical discs, objective lensdriving apparatuses for use with the reproduction systems of thoseoptical information recording mediums are actively being developed.Objective lens driving apparatuses have been widely used as compact disc(CDs) or CD-ROM driving apparatuses.

Recently, objective lens driving apparatuses have been developed for notonly reproducing use but also recording use. Well-known recordingmethods used for those apparatuses are the magneto-optical recordingmethod and the phase-modulation recording method. Many of such recordingmethods have been prescribed in detail by standards. In recent years,however, a high-density recording optical disc aimed at improvements inthe recording density has appeared and the research and development ofoptical discs of the high-density recording type have been done at arapid rate. With such optical discs, for high-density recording, pitsserving as data recording units are required to be made smaller thanthose on conventional CDs and the pits are needed to be searched forwith high accuracy. An optical disc of the high-density recording typediffers from a conventional CD in the thickness of the substrate. For anapparatus for reproducing the optical disc, the wavelength of a laserbeam searching for pits is shorter and the numerical aperture of theobjective lens is set larger so that the diameter of the beam spotformed on the optical disc may be smaller.

When various modification are made on new discs appearing one afteranother as described above, it is difficult for apparatuses of theabove-described type to record and reproduce the data onto and fromoptical discs complying with the conventional standards. It is aninconvenience to the users to prepare a disc apparatus according to therecording medium used.

To solve such a problem, a method of providing a plurality of opticalheads with different focal lengths on a single optical disc apparatushas been proposed, as disclosed in U.S. Pat. No. 5,235,581. With thedisc apparatus, two optical heads are provided so as to enable trackingindependently, thereby making it possible to record and reproduce thedata onto and from a conventional compact disc, such as a compact disc.

In such a method, two optical heads are positioned so that they may faceeach other symmetrically with the center of the optical disc and the twooptical heads cannot be placed side by side. Therefore, with an opticaldisc apparatus employing such a method, in the case of an optical disc(e.g., a CD ROM or MO) housed in a cartridge (caddie) with a windowsection, none of the two objective lenses can be positioned under theopening of the window section with a limited area. As optical discapparatuses have been popularized, there have been great demands towardlower-cost apparatuses. The need for two optical heads, however, is anobstacle to such demands.

From this viewpoint, it is hoped that an optical head in which two ormore objective lenses of different types are provided and which switchesthe objective lens therein will appear. With this configuration, thedevelopment of an optical head with a structure that can identify thetype of the selected objective lens is also wanted.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an objective lensdriving apparatus which can not only selectively switch between anobjective lens of a type capable of recording and reproducing the dataonto and from widely used optical information recording mediums and anobjective lens of a type capable of recording and reproducing the dataonto various types of optical information recording mediums expected toappear in the future, but also identify the type of the objective lensswitched and selected.

Another object of the present invention is to provide an objective lensdriving apparatus which includes at least two objective lenses withdifferent numerical apertures according to optical information recordingmediums conforming to different standards, and has a simpleconfiguration that can not only switch the objective lens according tothe optical information recording medium but also identify the type ofthe objective lens switched and selected.

According to the present invention, there is provided an apparatus forsearching a recording medium using a light beam. The apparatus comprisesa first objective lens and a second objective lens each having anoptical axis. A light beam generator generates a light beam directedalong any optical path extending between the light beam generator andthe recording medium. A lens holder having a rotational axis supportsobjective lens and second objective lens. A reflecting element isassociated with on the lens holder. The reflecting element is positionedand configured to reflect a part of the light beam generated by thelight beam generator and directed along the optical path. The lensholder is movable axially along the rotational axis thereof to therebymove one of the first and second objective lenses axially along itsrespective optical axis. An objective lens driver is constructed andarranged to rotate the lens holder to thereby select one of theobjective lenses by placing the selected objective lens in the opticalpath of the light beam generated by the light beam generator so that theselected objective lens focuses the light beam on the recording medium.The objective lens driver is constructed and arranged to move the lensholder axially along the optical axis of the selected objective lens tothereby adjust to focusing of the light beam by the selected objectivelens and rotate the lens holder to thereby cause the objective lens totrack a desired area of the recording medium. An objective lensidentifier is constructed and arranged to sense the part of the lightbeam reflected by the reflecting element and identify with one of theobjective lenses has been selected and placed in the optical path of thelight beam.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematic block diagram of an optical disc apparatusaccording to an embodiment of the present invention;

FIG. 2 is a detailed block diagram of the disc drive unit shown in FIG.1;

FIG. 3 is a schematic perspective view of the optical disc shown in FIG.1;

FIG. 4 is a plan view of an objective lens driving apparatus thatswitches and drives the objective lens shown in FIG. 2;

FIG. 5 is a perspective view of the optical pickup of the objective lensdriving apparatus shown in FIG. 4;

FIG. 6 is a sectional view of the internal structure of the lens holdersupport of the optical pickup shown in FIG. 5;

FIG. 7 is a perspective view of the lens holder of the optical pickupshown in FIG. 5;

FIG. 8 schematically shows the optical pickup of FIG. 5 and an opticalsystem related to the optical pickup;

FIG. 9 is a conceptual diagram to help explain the principle of floatingthe lens holder magnetically in the optical pickup shown in FIG. 5;

FIGS. 10A to 10F are perspectives view to help explain the principle offloating the lens holder magnetically in the arrangement of FIG. 9;

FIG. 11 is a perspective view of a magnetic circuit for switching theobjective lens in the optical pickup shown in FIG. 5;

FIG. 12 is a waveform diagram of a signal that causes the magneticcircuit of FIG. 11 to switch the objective lens;

FIGS. 13A and 13B are plan views to help explain the objective lensswitching operation by the objective lens driving apparatus;

FIGS. 14A and 14B are sectional views to help explain the operation ofjudging whether or not which objective lens is located in the laseroptical path in the optical pickup shown in FIG. 5;

FIG. 15 is a block diagram of a circuit that switches the driving systemfor the optical pickup on the basis of the judging operation of FIGS.14A and 14B;

FIG. 16 is a schematic plan view of a shutter built in a disc apparatusaccording to another embodiment of the present invention;

FIGS. 17 and 18 schematically show the optical systems of disc driveunits in which the shutter shown in FIG. 16 has been built;

FIG. 19 is a plane view showing modifications of the optical pickup ofthe objective lens driving apparatus, shown in FIG. 4, which havedifferent sensor mechanisms for sensing the lens position; and

FIGS. 20, 21, 22 and 23 are perspective views showing modifications ofthe optical pickup of the objective lens driving apparatus, shown inFIG. 4, which have different sensor mechanisms for sensing the lensposition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an optical disc reproducing apparatus provided with anobjective lens driving apparatus according to an embodiment of thepresent invention will be explained by reference to the accompanyingdrawings.

FIG. 1 is a block diagram of an optical disc reproducing apparatus thatreproduces the data from an optical disc associated with an embodimentof the present invention. FIG. 2 is a block diagram of the disc drivesection that drives the optical disc shown in FIG. 1. FIG. 3 illustratesthe structure of the optical disc shown in FIG. 2.

With the optical disc reproducing apparatus of FIG. 1, when the useroperates a key control and display section 4, the recording data, thatis, the video data, sub-video data, and audio data, is reproduced fromthe optical disc 10. The reproduced data is converted into a videosignal and an audio signal in the apparatus, which are supplied to amonitor section 6 and a speaker section 8, respectively. The monitorsection reproduces the image from the video signal and the speakersection reproduces the sound from the audio section.

There have been optical discs 10 with various structures. For instance,one commercially available optical disc of a high-density recording typeon which the data has been recorded at a high density is shown in FIG.3, a recording layer, or a pair of structural members 18 in which areflecting layer 16 is formed, is provided on a transparent substrate14, with the pair of structural members 18 being laminated together withan adhesive layer 20 so that the recording layer 16 may be sealed in it.The optical disc 10 thus constructed has a central hole 22 into whichthe spindle of a spindle motor 12 can be inserted. Around the centralhole 22, there is provided a clamping area 24 against which the opticaldisc 10 is pressed while the disc is rotating.

The area from the clamping area 24 to the outer edge of the optical disc10 is an information recording area 25 in which information can berecorded on the optical disc 10. The optical disc shown in FIG. 3 hasthe information area 25 on each side of the disk. For each informationrecording area 25, its outer periphery area is a lead-out area 26 inwhich no information is normally recorded and its inner periphery areaadjoining the clamping area 24 is a lead-in area 27 in which noinformation is normally recorded. The area between the lead-out area 26and the lead-in area 27 is a data recording area 28. At the recordinglayer 16 in the information recording area 25, tracks are formed in aspiral continuously as an area in which data is to be recorded. Thecontinuous tracks are divided into a plurality of sectors. Data isrecorded on the basis of the sectors. The data recording area 28 in theinformation recording area 25 is an actual data recording area, in whichthe management data, the main video data, the sub-video data, and theaudio data are recorded in the form of physical changes, such as pits.With the read-only optical disk 10, pit trains have been formed by astamper on the transparent substrate 14. A reflecting layer is formed onthe side of the transparent substrate 14 on which the pit trains havebeen formed by evaporation. The reflecting layer is used as a recordinglayer 14. In the read-only optical disk 10, grooves are not particularlymade and the pit trains are tracks. Usually, such a high-densityrecording type of optical disc 10 has a transparent substrate 14 with athickness of 0.6 mm, half of the thickness of the transparent substrateof a conventional optical disc, such as a CD or a CD-ROM, whosetransparent substrate has a thickness of 1.2 mm.

With the optical disc 10 is loaded in a disc drive section 30 of theoptical disc reproducing apparatus that reproduces the data from theoptical disc 10. The disc drive section 30 drives the optical disc andsearches the optical disc 10 using a light beam. Specifically, as shownin FIG. 2, the optical disc 10 is placed on a spindle motor 12 driven bya motor driving circuit 11 and is rotated by the spindle motor 12. Underthe optical disc 10, an optical head, or an optical pickup 32, thatfocuses the light beam, or the laser beam, on the optical disc 10 isprovided. The optical pickup 32, which will be explained in detaillater, includes an objective lens 35 with a small numerical aperture forCD or CD-ROM and an objective lens 34 with a large numerical aperturefor a high-density recording type of optical disc explained in FIG. 3.The optical pickup further includes an objective lens switching circuit39 that generates a driving signal for switching between the objectivelenses 34 and 35. When the type of the optical disc 10 to be searched,that is, whether the optical disc is of a conventional CD type or ahigh-density recording type, is determined, the objective lens switchingcircuit 39 operates to generate a driving signal. By the driving signalfrom the objective lens switching circuit 39, one of the objectivelenses 34 and 35 is selected according to the determined type of opticaldisc 10 and is placed in the laser beam optical path.

The optical head 32 is placed on a guide mechanism so as to be able tomove radially along the optical disc 10 to search for the informationrecording area, particularly the data recording area 28, and is movedradially along the optical disc 10 by a feed motor 33 driven by thedriving signal from a driving circuit 37. In the optical disc apparatus,the objective lenses 34, 35 are supported in a manner to allow movementalong their optical axes and are moved along their optical axes inresponse to the driving signal from a focus driving circuit 36 so thatthe objective lenses 34, 35 is always in focus, thereby forming a verysmall beam spot on the recording layer 16. Furthermore, as explained indetail later, the objective lenses 34, 35 are supported so as to moveminutely along the radius of the optical disc 10 and are moved minutelyin response to the driving signal from a track driving circuit 38 so asto be always kept in the tracking state, thereby causing the light beamto trace the tracks on the recording layer 16 of the optical disc 10.

The optical head 32 senses the light beam reflected by the optical disc10 and supplies the sense signal to a servo processing circuit 44 via ahead amplifier 40. The servo processing circuit 44 generates a focussignal, a tracking signal, and a motor control signal from the sensesignal and supplies these signals to the focus driving circuit 36, trackdriving circuit 38, and motor driving circuit 11, respectively. As aresult, the objective lenses 34, 35 are kept in the in-focus state andin the tracking state. Furthermore, the spindle motor 12 is rotated at aspecific number of revolutions and the light beam traces a track on therecording layer 16 at, for example, a constant linear speed. When asystem CPU section 50 supplies a control signal, or an access signal, tothe servo processing circuit 44, the servo processing circuit 44supplies a move signal to the driving circuit 37 and the optical head 32is moved radially with respect to the optical disc 10 and accesses aspecific sector on the recording layer 16. The reproduced data isamplified by the head amplifier 40. The amplified signal is outputtedfrom the disc drive section 30.

The outputted reproduced data is stored in a data RAM section 56 via thesystem CPU section and a system processor section 54 which arecontrolled by the programs stored in a system ROM and RAM section 52.The stored reproduced data is processed by the system processor section54, which classifies the data into video data, audio data, and sub-videodata. The video data, audio data, and sub-video data are supplied to avideo decoder section 58, an audio decoder section 60, and a sub-videodecoder section 62, respectively, which decode the respective data. Thedecoded video data, audio data, and sub-video data are converted by aD/A and reproducing circuit 64 into an analog video signal, analog audiosignal, and analog sub-video signal. The D/A and reproducing circuitalso performs a mixing process on the decoded video data, audio data,and sub-video data and supplies a video signal and a sub-video signal tothe monitor 6 and an audio signal to the speaker 8. As a result, themonitor section 6 displays the image and the speaker section reproducesthe sound.

The details of the optical pickup 32 of FIG. 2 and its guide mechanismwill be described by reference to FIGS. 4 to 11.

The spindle motor 3 is fixed to a base 71 as shown in FIG. 4. Theoptical disc 10 rotated by the spindle motor 3 is held by chuck means(not shown). Under the optical disc 10, a pair of guide rails 73 placedradially parallel to the disc is secured to the base 71. On the guiderails 73, a carriage 72 that moves along the guide rails 73 is placed.An objective lens actuator shown in FIG. 5 is provided on the carriage72.

The objective lens actuator shown in FIG. 5 is composed of a lens holder75 capable of floating and rotating and a lens holder support 74 inwhich the lens holder 75 is housed. An actuator base 76 that is fixed tothe carriage 72 and has an opening section 78 for a laser beam opticalpath is provided on the lens holder support 74. In the central portionof the actuator base 76, a shaft 77 is secured. In the support 74, anarc-shaped yoke 79 is secured to the actuator base 76 along thecircumference around the shaft 77. In the arc-shaped yoke 79, two pairsof arc-shaped permanent magnets 81, 82 are placed symmetrically aroundthe shaft 77, with a set of magnets facing each other being magnetizedin the same direction. One set of permanent magnets 81 is magnetized sothat the north (N) pole and the south (S) pole may be arranged along theshaft 77 as shown in FIG. 6. The other set of permanent magnets 82 ismagnetized along the arc of the circular arc of the arc-shaped yoke 79as shown in FIG. 6.

The lens holder 75 is formed into a roughly circular cylinder as shownin FIG. 7. On the top surface of the lens holder, the objective lens 35for CD or the like and the objective lens 34 for a high-densityrecording type are provided. Under the objective lenses 34, 35, a cavityis provided so as to allow laser beams to pass through. The objectivelenses 34, 35 are fixed to the lens holder 75 so that their optical axesmay be located on the same circumference around the center of the lensholder 75. In the center of the lens holder 75, bearings 83 into whichthe shaft 77 is inserted are fixed. The bearings 83 enables the shaft 77to support the lens holder 75 so that the holder may rotate and make anup-and-down movement. Around the lens holder 75, magnetic materials 84are embedded symmetrically with respect to the shaft 77. On the magneticmaterials 84, four magnetic coils 85, 86 are secured and arrangedsymmetrically with respect to the shaft 77.

At the side of the lens holder 75 near the objective 35, a reflectingelement 92, for example, a reflecting mirror or a reflecting prism, isprovided as shown in FIG. 7. The reflecting element 92 is positioned inthe optical path of the laser beam so that part of the laser beamheading for the objective lens 35 may enter the reflecting element. Asshown in FIG. 8, a sensor 100 that senses part of the light beamreflected by the reflecting element 92 is provided in the lens holdersupport 74. When one of the objective lenses 34, 35 is selected and thereflecting element 92 is positioned in the laser beam optical path asdescribed later, part of the laser beam is reflected by the reflectinglens 92 and is sensed by the sensor 100. Aperture stops that limit thelaser beam entering the objective lenses 34, 35 are usually provided onthe light source side of the objective lenses 34, 35. The reflectingelement 92 is placed on the light source of the aperture stops so as toreflect part of the laser beam that should have been limited by theaperture stops. From the viewpoint of placing the reflecting element 92so as to reflect part of the laser that should have been limited by theaperture stops, it is desirable that the reflecting element 92 should beprovided on the side of the lens holder 75 near the objective lens 35with the small aperture for reproducing a CD or the like in terms ofspace. The reflecting element is not limited to a separate element. Forinstance, it may be such that part of the lens holder 75 is formed at areflecting surface or that part of the aperture stop is formed into areflecting surface.

The optical pickup 32 and an optical unit 90 of the optical systemrelated to the optical pickup 32 are shown in FIG. 8. The optical unit 9including a semiconductor laser 94 that generates a laser beam focusedon the optical disc 10 is housed and secured in the internal space ofthe carriage 72, a movable body. The laser beam generated by thesemiconductor laser 94 in the optical unit 90 is collimated by thecollimator lens 91. The collimated beam is reflected by a beam splitter93 and the resulting beam is directed outside the optical unit 90. Thelaser beam from the optical unit 90 is directed to either objective lens34 or objective lens 35 in the optical pickup 32 fixed on the carriage72. Either objective lens 34 or objective lens 35 focuses the laser beamon a recording track on the optical disc 10. The laser beam reflected bythe optical disc 10 passes through one of the objective lenses 34, 35again and is returned to the optical unit 90. In the optical unit 90,the laser beam passes through the beam splitter 93 and is separated by abeam splitter 95 into two sub-beams. The respective beams are gatheredby condenser lenses 96, 97. These condensed beams are sensed by a firstphotodetector 98 and a second photodetector 99 provided in the opticalunit 5. Using the sense signals from the photodetectors 98, 99, aninformation signal, focus error signal, track error signal, etc. aregenerated as described earlier. Use of the focus error signal enablesthe positional error of the selected one of the objective lenses 34, 35in the focus direction to be sensed. To correct the positional error,current is supplied to one of the coils 85, 86 as explained later. Inaddition, use of the track error signal enables the positional error ofthe objective lenses 34, 35 in the track direction to be sensed. Currentis supplied to the other of the coils 85, 86 so as to correct thepositional error. In this way, the information is recorded onto arecording track on the optical disc 10 and the information is read fromthe recording track on the optical disc 19.

The details of the operation of the optical pickup 32 will be explained.

First, the reason why the lens holder 75 is floated magnetically by whatis called a magnetic spring within the lens holder support 74. Asdescribed earlier, in the lens holder 74, two pairs of permanent magnets81, 82 are arranged symmetrically around the shaft 77 of the lens holdersupport 74. On each of the permanent magnets 81, 82, magnetic materials84 are placed so as to face each other with a gap between them.Specifically, the magnetic materials 84 are arranged symmetricallyaround the shaft 77 and are secured to the lens holder 75. Consequently,the magnetic materials 84 are attracted by the permanent magnets 81, 82in such a manner that the permanent magnets 81, 82 and magneticmaterials 84 are kept in a neutral position or in a stable state, asshown in FIGS. 10A and 10B. As a result, the lens holder 75 is floatedmagnetically within the lens holder support 74. In this situation, whendisturbance is given to the lens holder 75 and the magnetic material 84has deviated upward from the neutral position as shown in FIG. 10C, thedownward force to put the magnetic material 84 back to the neutralposition greater than the upward force is exerted on the magneticmaterial 84, so that the magnetic material 84 is returned to the neutralposition. Similarly, when the lens holder 75 is disturbed and themagnetic material 84 has deviated downward from the neutral position asshown in FIG. 10E, the upward force to put the magnetic material 84 backto the neutral position greater than the downward force is exerted onthe magnetic material 84, so that the magnetic material 84 is returnedto the neutral position. Furthermore, when the lens holder 75 isdisturbed and the magnetic material 84 has deflected rightward along thecircumference from the neutral position as shown in FIG. 10D, theleftward force to put the magnetic material 84 back to the neutralposition greater than the rightward force is exerted on the magneticmaterial 84, so that the magnetic material 84 is returned to the neutralposition. Similarly, when the lens holder 75 is disturbed and themagnetic material 84 has deflected leftward along the circumference fromthe neutral position as shown in FIG. 10F, the rightward force to putthe magnetic material 84 back to the neutral position greater than theleftward force is exerted on the magnetic material 84, so that themagnetic material 84 is returned to the neutral position.

The magnetic materials 84 are arranged in axisymmetric positions. Whenthe lens holder 85 is rotated and the objective lens is switched asexplained later, magnetic attraction causes the position of the firstobjective lens 34 in the neutral position to coincide with the neutralposition of the second objective lens 35, so that the second objectivelens 35 can be used in the state where the optical unit 90 and the firstobjective lens 34 have been adjusted.

Now, the switching operation for selecting either objective lens 34 orobjective lens 35 will be explained. With the objective lens 34 with thelarge numerical aperture being put in the optical path of the laser beamas shown in FIG. 7 and FIG. 13A, the coil 85 is assumed to face thepermanent magnet 82 magnetized in the circumferential direction and thecoil 86 is assumed to face the permanent magnet 81 magnetized in theaxial direction. This state corresponds to the neutral state alreadydescribed in which the lens holder 75 is kept in place stably. In thestable state, when a positive current as shown by arrow P0 is suppliedto the coil 85 at time t1 as shown in FIG. 12, the axial directionportions 85A, 85B of the coil 85 parallel to the axis 77 are suppliedwith a current interacting with a magnetic field produced by thepermanent magnet 82, which produces force FR that generates torque inthe circumferential direction, thereby causing the lens holder 75 tostart to rotate. During the interval between time t1 to time t2,starting force that rotates the lens holder 75 sufficiently is exertedon the coil 85. At time t2 that the coil 85 has begun to rotate and thecoil portion 85B on the outgoing side of the coil 85 faces the south (S)pole of the permanent magnet 82, the current supplied to the coil 85 isinverted as shown in FIG. 12. The inversion produces torque FR thatmoves the coil 85 away from the permanent magnet 82 between the coilportion 85B on the outgoing side of the coil 85 and the S pole of thepermanent magnet 82. The torque is applied to the coil 86. As a result,the coil 86 is rotated toward the front side of the permanent magnet 81.At time t3 in the course of rotation, the supply of current to the coil86 is stopped. After time t3, the lens holder 75 rotates under itsinertia. Although the coil 86 passes the neutral point of the permanentmagnet 81 temporarily, the coils 85, 86 are returned to the stableneutral position according to the principle explained in FIGS. 10A to10F. In this way, the rotation of the lens holder 75 causes the coil 86to face the permanent magnet 82 and the coil 85 to face the permanentmagnet 81 and places the objective lens 35 with the small numericalaperture in the optical path of the laser beam in place of the objectivelens 34 with the large numerical aperture, which virtually switches theobjective lens.

In a case where the lens holder 75 is rotated and the objective lenses34, 35 are thereby switched, when the clearance between the rotatingshaft 77 and the bearings 83 is set at 10 microns or less, theinstallation position error between the first objective lens 34 and thesecond objective lens can be ignored.

Now, the focusing operation and tracking operation of the optical pickup32 of FIG. 5 will be explained.

With the objective lens 34 with the large numerical aperture being putin the optical path of the laser beam as shown in FIG. 7 and FIG. 13A,the coil 86 facing the permanent magnet 81 magnetized in the axialdirection for focus control acts as a focus control coil and the coil 85facing the permanent 82 magnetized along the circumference for trackingcontrol acts as a tracking control coil. Specifically, when a focus coildriving current Fi is supplied to the coil 86 in response to the focuserror signal, the circumferential direction portions 86A, 86B of thecoil 86 interact with a magnetic field produced by the permanent magnet81, which causes upward or downward force to act on the coil 86according to the direction of the current Fi, thereby moving the lensholder 75 upward or downward along the axial direction and keeping theobjective lens 34 in the in-focus state. When a tracking coil drivingcurrent Ti is supplied to the coil 85 in response to the tracking errorsignal, the axial direction portions 85A, 85B of the coil 85 interactwith a magnetic field produced by the permanent magnet 82, which causesrightward or leftward force to act on the coil 85 according to thedirection of the current Ti, thereby rotating the lens holder 75 in thecircumferential direction and keeping the objective lens 34 in theon-track state.

As explained earlier, after switching to the objective lens 35, theobjective lens 35 with the small numerical aperture is put in theoptical path of the laser beam as shown in FIG. 13B. In this state, thecoil 85 facing the permanent magnet 81 magnetized in the axial directionfor focus control acts as a focus control coil and the coil 86 facingthe permanent 82 magnetized along the circumference for tracking controlacts as a tracking control coil. Specifically, when a focus coil drivingcurrent Fi is supplied to the coil 85 in response to the focus errorsignal, the circumferential direction portions 85C, 86D of the coil 85interact with a magnetic field produced by the permanent magnet 81,which causes upward or downward force to act on the coil 85 according tothe direction of the current Fi, thereby moving the lens holder 75upward or downward along the axial direction and keeping the objectivelens 34 in the in-focus state. When a tracking coil driving current Tiis supplied to the coil 86 in response to the tracking error signal, theaxial direction portions 86C, 86D of the coil 86 interact with amagnetic field produced by the permanent magnet 82, which causesrightward or leftward force to act on the coil 86 according to thedirection of the current Ti, thereby rotating the lens holder 75 in thecircumferential direction and keeping the objective lens 34 in theon-track state.

As described above, with the objective lens driving apparatus of thepresent invention, the coils that perform a tracking operation withoutexternally applied force switches objective lens 34 to objective lens 35or vice versa, there is no possibility that excessive force will act onthe objective lenses and incline their optical axes, enabling thereproduction of a stable signal. When the coils 81, 82 switch betweenthe objective lenses 34, 35, this changes the role of the objective lensfrom tracking operation to focusing operation or vice versa, resultingin not only the improved utilization of the coils but also the improveddriving sensitivity.

Since the objective lenses 34, 35 switched by the same coil are foreither tracking operation or focusing operation, depending on thesituation, it is possible to verify which objective lens is in usewithout an additional sensing device by forcing current to flow througheither coil 81 or coil 32 and sensing the direction of movement of theobjective lenses 34, 35. The verifying operation will be explained byreference to FIGS. 14A and 14B.

FIGS. 14A and 14B show the relationship between the reflecting element92 and the sensor 100 which judges which objective lens is placed in thelaser optical path in the optical pickup of FIG. 5. As shown in FIGS. 5,7, and 8, the reflecting element 92 is assumed to be positioned near theobjective lens 35 with the small aperture. As shown in FIG. 14A, whenthe lens holder 75 is rotated and the objective lens 35 is put in theoptical path of the laser beam, part of the laser beam is reflected bythe reflecting element 92. The reflected part of the laser beam issensed by the sensor 100. When the sensor 100 outputs a sense signal, itis judged that the CD objective lens 35 with the small aperture is inthe effective state and the CD objective lens 35 is in the laser beamoptical path. In contrast, as shown in FIG. 14B, when the objective lens34 with the large aperture for high-density recording optical discs ispositioned in the optical path of the laser beam, part of the laser beamis not reflected toward the sensor 100 because the reflecting element 92is not provided near the objective lens 34, so that the laser beam isnot sensed by the sensor 100. Accordingly, the sensor 100 does notoutput a sense signal, from which it is judged that the objective lens34 with the larger aperture for high-density recording optical discs isin the effective state and the objective lens 34 for high-densityrecording discs is in the laser beam optical path.

FIG. 15 is a block diagram of a circuit that switches the drive circuitaccording to the objective lens switching signal. In the circuit shownin FIG. 15, an objective lens identifying circuit 104 that generates anidentification signal for identifying the types of the objective lenses34, 35 according to the presence or absence of the sense signal from thesensor 100 is connected to the sensor 100 and the identification signalfrom the objective lens identifying circuit 104 is inputted to the CPU50 as the need arises. When the objective lens identifying circuit 104supplies the identification signal to the system CPU section 50, theidentification signal is compared with the medium type identificationsignal from the key operation and display section 4. The medium typeidentification signal indicates whether the medium to be loaded is anoptical disc of the high density type or an ordinary optical disc, suchas a CD. When the type of the medium coincides with the type of theobjective lens, the reproducing operation is started. When they do notcoincide with each other, the objective lens switching signal of FIG. 12is generated, the type of objective lens is switched.

In the circuit of FIG. 15, a focus error signal generator circuit 103, afocus servo circuit 105, one of coils 85 and 86, and one of drivingcircuits 106 and 108 corresponding to the coil constitute a focus servoloop. A tracking error signal generator circuit 111, a track servocircuit 107, one of coils 85 and 86, and one of driving circuits 106 and108 corresponding to the coil constitute a tracking servo loop. In thecircuit of FIG. 15, a servo loop switching circuit 110 is providedbetween the focus servo circuit 105 and track servo circuit 107 andbetween the driving circuit 106 and driving circuit 108. In response tothe identification signal from the objective lens identifying circuit104 that identifies the types of the objective lenses 34, 35 asdescribed earlier, the servo loop switching circuit 110 switches theconnection so that the CPU 50 may form a suitable servo loop.Specifically, when the coil 85 functions as a focus coil, the signalfrom the CPU 50 activates the servo loop switching circuit 110, whichconnects the driving circuit 106 connected to the coil 85 to the focusservo circuit 105 and the driving circuit 108 connected to the coil 86to the tracking servo circuit 107. Similarly, when the coil 85 functionsas a tracking coil, the signal from the CPU 50 activates the servo loopswitching circuit 110, which connects the driving circuit 106 connectedto the coil 85 to the tracking servo circuit 107 and the driving circuit108 connected to the coil 86 to the focusing servo circuit 108.

In the objective lens switching and driving apparatuses, if the numberof objective lenses is n, it is desirable that a 2n number of permanentmagnets and a 2n number of coils should be arranged circumferentially toform a magnetic circuit. In this arrangement, the coils and permanentmagnets facing each other form a magnetic circuit for focusing andtracking control, which exerts equal force on the lens holder infocusing control and tracking control, enabling the lens holder to bedriven in a well-balanced manner with high accuracy. That is, not onlythe vibration characteristic but also the driving characteristic areimproved.

As explained until now, according to the present invention, in anobjective lens driving apparatus where objective lenses of differenttypes are fixed to a lens holder and the lens holder is rotated forselection of an objective lens, a reflecting element that reflects partof the laser beam heading for one objective lens is provided on the lensholder near the objective lens and a sensor that senses the presence orabsence of the reflected light beam from the reflecting element isprovided. This enables the type of the objective lens put in the laseroptical path to be identified easily according to the presence orabsence of the sense signal from the sensor. According to theidentification result, the optical pickup is controlled suitably.

While in the above embodiment, the objective lens 34 is switched to theobjective lens 35 or vice versa, the present invention may be applied toa system that projects a laser beam on the optical disc 10 with theoptimum numerical aperture obtained by combining a common objective lens34 with a large diameter with apertures 54 and 59. Specifically, theinvention may be applied to an apparatus as shown in FIG. 16 where ashutter 60 having a small aperture 59 and a large aperture 54 isprepared and the shutter 60 is inserted between the beam splitter 93 andthe objective lens 34 as shown in FIGS. 17 and 18. In this apparatus,the reflecting mirror 92 is fixed on the shutter 60 near the smallaperture 59.

With such an apparatus, the aperture selecting mechanism 55 moves theshutter 60 and the large aperture 54 is selected, the optical sensor 100does not sense the light beam because the reflecting mirror 92 is not inthe optical path of the light beam heading for the aperture 54 as shownin FIG. 17. Therefore, the sensor 100 does not generate a sense signal,from which it is judged that the objective lens 34 with the largeaperture for high-density recording optical discs is in the effectivestate and that the objective lens 34 with the numerical aperture forhigh density optical discs is in the laser beam optical path. Incontrast, when the aperture selecting mechanism 55 moves the shutter 60and the small aperture 59 is selected, the reflecting mirror 92 is putin the optical path of the light beam heading for the aperture 59 asshown in FIG. 19, so that the reflecting mirror 92 reflects part of thelight beam toward the optical sensor 100. Therefore, the sensor 100generates a sense signal, from which it is judged that the objectivelens 34 with the small aperture for CDs is in the effective state andthat the objective lens 34 with the numerical aperture for CDs is in thelaser beam optical path.

As described above, the reflecting element that reflects part of thelaser beam heading for the objective lens is provided on the shutter andthe sensor that senses the presence or absence of the reflected lightbeam from the reflecting element is provided. This makes it possible toeasily identify the effective aperture of the objective lens put in thelaser optical path according to the presence or absence of the sensesignal from the sensor. According to the identification result, theoptical pickup is controlled suitably.

In the above embodiment, part of the laser beam heading for theobjective is sensed to judge the type of the objective. The presentinvention is not restricted to the optical sensing means, and may beapplied to other sensing means that identify the type of the objective.Hereinafter, various examples of sensing means will be explained asother embodiments of the present invention.

In FIG. 19, a mechanical switch is employed as a sensing mechanism thatsenses the lens position. Specifically, as shown in FIG. 19, on the topsurface of a lens holder 75, a stylus 200 made of silicone rubber isprovided, for example. The tip 201 of the stylus 200 is such that whenone of the objectives 34 and 35 has been selected, the tip comes intocontact with and presses the leaf spring 211 of a micro switch 210 thatis located outside the lens holder 75 and has the function of a contactposition sensor. When the other objective has been selected, the tip 201of the stylus 200 does not come into contact with the leaf spring 211,which is thus left open.

In the embodiment of FIG. 19, the stylus 200 is positioned on a diagonalextending from the objective 35 having an almost small aperture withrespect to the rotational center of the lens holder 75. Therefore, whenthe lens holder 75 is rotated and the objective 35 is placed in thelaser beam optical path, the tip 201 of the stylus 200 on the topsurface of the lens holder 75 will come into contact with the leafspring 211 of the micro switch 210 that is provided outside the lensholder 75 and functions as a contact position sensor and will press theleaf spring 211. When the leaf spring 211 is pressed, the switch of thelens position sensing circuit 210 will be closed, the small-apertureobjective 35, or, CD objective 35, be located in a specific position,and a sense signal indicating that the small-aperture objective 35 hasbeen selected and is in the effective state will be supplied to theobjective identifying circuit 104. When the large-aperture objective 34is placed in the laser beam optical path in place of the small-apertureobjective 35, the tip 201 of the stylus 20 will be separatedsufficiently from the leaf spring 211 of the micro switch 210, bringingthe switch of the lens position sensing circuit 210 into an open state.As a result, a sense signal indicating that the small-aperture objective35 is effective is not outputted and the fact that the relativelylarge-aperture objective 34 for DVD is identified the an objectiveidentifying circuit 104. It has been known that micro switches, whosestandards are prescribed by JIS C4505, are miniature switches with asmall interval between contacts and provide open/close operation of thecircuit through mechanical operation. In addition, micro switches havean operation mechanism that causes the variable contact to move from onefixed contact point to the other at high speed, regardless of the speedat which the switch is operated.

On the other hand, the stylus 200 is made of, for example, relativelyflexible material, such as silicone rubber, and has its shape anddimensions optimized. Specifically, the stylus has a suitable springproperty that applies pressure reliably to the micro switch 210. Sincethe micro switch is of the type responsive to very low pressure, evenwhen the tip 201 is in contact with the leaf spring 211 of the microswitch 210, it has no adverse effect on the vibration characteristic ordriving characteristic of the lens holder 75. Because of the springproperty, when the objective 35 has been selected, the tip 201 continuespressing the leaf spring 211 of the micro switch 210 stably.

The micro switch is of the type that has been built in apparatuses invarious fields and has achieved good results in terms of reliability andstability. Micro switches of this type whose unit price is low arecommercially available. Therefore, even when the micro switch is builtin the above-described sensing mechanism, a highly reliable, highlystable lens identifying mechanism can be realized without raising thecost of the apparatus.

While in the above embodiment, the sensing mechanism using a microswitch has been explained, the present invention is not limited to themicro switch. The invention may be applied to another sensing mechanism,as long as the sensing mechanism is an element that mechanically sensesthe pressure produced by the rotation of the lens holder.

In FIG. 20, a mechanism that senses a change in electrical resistance isused as a sensing mechanism that senses the lens position. Specifically,as shown in FIG. 20, on the top surface of the lens holder 75, aconductive stylus 300 made of thin phosphor bronze is provided. When oneof the objectives 34 and 35 has been selected, the tip 301 of the stylus300 is brought into contact with an electrical resistance element 310located outside the lens holder 75. When the other objective has beenselected, the tip 301 of the conductive stylus 300 is not brought intocontact with the electrical resistance element 310.

In the embodiment of FIG. 20, the conductive stylus 300 is positioned ona diagonal extending from the objective 35 having an almost smallaperture with respect to the rotational center of the lens holder 75.Therefore, when the lens holder 75 is rotated and the objective 35 isplaced in the laser beam optical path, the tip 301 of the stylus 300 onthe top surface of the lens holder 75 will come into contact with theelectrical resistance element 310 provided outside the lens holder 75.The conductive stylus 300 is made of thin phosphor bronze. A lead wire302 is connected to one end of the stylus. A lead wire 312 is connectedto one end of the electrical resistance element 310. Both of the leadwires 301, 312 are connected to a lens position sensing circuit 210.When the tip 301 of the conductive stylus 300 is brought into contactwith the electrical resistance element 310, the conductive stylus 300comes into electrical contact with the electrical resistance element310, allowing the sense current to flow. The current is sensed at thelens position sensing circuit 210. As a result, the small-apertureobjective 35, or CD objective 35, is placed in a specific position and asense signal indicating that the small-aperture objective 35 has beenselected and is in the effective state is supplied to the objectiveidentifying circuit 104. When the large-aperture objective 34 is placedin the laser beam optical path instead of the small-aperture objective35, the tip 301 of the conductive stylus 300 is separated sufficientlyfrom the electrical resistance element 310 and the lens position sensingcircuit 210 is brought into the off state, preventing the sense currentfrom being sensed. As a result, the sense signal indicating that thesmall-aperture objective 35 is effective is not outputted and the factthat the relatively large-aperture objective 34 for DVD is effective isrecognized by the objective identifying circuit 104.

On the other hand, the conductive stylus 300 is made of relativelyflexible material and has its shape and dimensions optimized.Specifically, the stylus has a suitable spring property that appliespressure reliably to the electrical resistance element 310. Therefore,even when the tip 301 is in contact with the electrical resistanceelement 310, it has no adverse effect on the vibration characteristic ordriving characteristic of the lens holder 75. Because of the springproperty, when the objective 35 has been selected, the tip 301 is keptin contact with the electrical resistance element 310 stably. Since theelectrical resistance element 310 is fixed to a yoke 79 via aninsulating adhesive 311, this prevents unnecessary current from flowingthrough the other parts even when current is allowed to flow through theelectrical resistance element 310.

Both of the conductive stylus 300 and the electrical resistance element310 are simple in structure and low in unit price. Therefore, even whenthe conductive stylus 300 and electrical resistance element 310 arebuilt in the above-described sensing mechanism, this neither raises thecost of the apparatus nor makes the apparatus larger. By building theconductive stylus 300 and electrical resistance element 310 in thesensing mechanism, a highly reliable, highly stable lens identifyingmechanism can be realized.

While in the present embodiment, the sensing mechanism in which theconductive stylus 300 and electrical resistance element 310 have beenbuilt has been explained, the sensing mechanism is not restricted to theconductive stylus 300 and electrical resistance element 310. As long asan element senses the pressure produced by the rotation of the lensholder in the form of a change in electrical resistance, the element maybe used as the sensing mechanism.

In FIG. 21, a sensing mechanism based on the principle ofelectromagnetic induction is used as a sensing mechanism that senses thelens position. Specifically, as shown in FIG. 21, on the top surface ofthe lens holder 75, a thin plate 400 made of metal, such as aluminum, isprovided. When one of the objectives 34 and 35 has been selected andplaced in the optical path, the top surface 410 of the thin plate 400 isforced to face the tip 411 of an eddy current loss-type minutedisplacement sensor 410 located outside the lens holder. When the otherobjective has been selected, the top surface 401 of the thin plate 400is moved to a place separate from the displacement sensor 410.

In the embodiment of FIG.. 21, the thin plate 400 is positioned on adiagonal extending from the objective 35 with an almost small apertureto the rotational center of the lens holder 75. Therefore, when the lensholder 75 is rotated and the objective 35 is placed in the laser beamoptical path, the top surface 401 of the thin plate 400 provided on thetop surface of the lens holder 75 is forced to face the tip 411 of theeddy current loss-type minute displacement sensor 410 outside the lensholder 75. The top surface 401 of the thin plate 400 and the tip 411 ofthe eddy current loss-type minute displacement sensor 410 are designedto stay out of contact with each other and maintain a spacing thatassures the generation of a specific eddy current as a result of theoperation of the focusing servo and tracking servo, even when the lensholder 75 moves up and down and rotates minutely. When the top surface401 of the thin plate 400 is forced to face the tip 411 of the eddycurrent loss-type minute displacement sensor 410, eddy current developsat the top surface 401 of the thin plate 400. The eddy current is sensedby the minute displacement sensor 410, which outputs the sensing resultas a sense signal. As a result, the small-aperture objective 35, or CDobjective 35, is placed in a specific position and a sense signalindicating that the small-aperture objective 35 has been selected and isin the effective state is supplied to the objective identifying circuit104. When the large-aperture objective 34 is placed in the laser beamoptical path instead of the small-aperture objective 35, the top surface401 of the thin plate 400 is separated sufficiently from the minutedisplacement sensor 410 and the minute displacement sensor 410 isbrought into the off state, preventing the sense current from beingsensed. As a result, the sense signal indicating that the small-apertureobjective 35 is effective is not outputted and the fact that therelatively large-aperture objective 34 for DVD is effective isrecognized by the objective identifying circuit 104.

It is known that the eddy current loss-type minute displacement sensor410 senses the displacement of a body or the presence or absence of abody using the principle of electromagnetic induction. Specifically,eddy current is generated in the metal to be sensed by electromagneticinduction and the loss of power in the coil is converted into a voltage,which is used as a sense signal. Such a sensor is of the non-contacttype, so that it has no effect on the lens holder 75 while the focusingservo and tracking servo are acting on the lens holder. Additionally, inthe present embodiment, neither the position nor displacement of theobject to be sensed need be measured with high accuracy, as long as thepresence or absence of the object to be sensed is sensed. Therefore, useof the minute displacement sensor 410 whose unit price is low and has alimited function will accomplish the purpose sufficiently. Therefore,even when the minute displacement sensor 410 is built in theabove-described sensing mechanism, a highly reliable, highly stable lensidentifying mechanism can be realized without raising the cost of theapparatus.

While in the above embodiment, the sensing mechanism incorporating theeddy current loss-type minute displacement sensor 410 has beenexplained, the present invention is not limited to the eddy currentloss-type minute displacement sensor 410. The invention may be appliedto another sensing mechanism, as long as the sensing mechanism is anelement that senses the rotation of the lens holder under the principleof electromagnetic induction, such as an element of the differentialtransformer type or a magnetic scale.

In FIG. 22, a sensing mechanism making use of a change in capacitance isused as the sensing mechanism that senses the lens position. As shown inFIG. 22, on the top surface of the lens holder 75, a thin plate 500 madeof material with a high permittivity is provided. When one of theobjectives 34 and 35 has been selected and the selected one is placed inthe optical path, the tip 501 of the thin plate 500 is forced tointervene in the gap 513 between the sensor sections 511, 512 of acapacitance-type minute displacement sensor 510 located outside the lensholder. When the other objective has been selected, the thin plate 500is moved to a place separate from the displacement sensor 510.

In the embodiment of FIG. 22, the thin plate 500 with a highpermittivity is positioned on a diagonal extending from the objective 35with an almost small aperture to the rotational center of the lensholder 75. Therefore, when the lens holder 75 is rotated and theobjective 35 is placed in the laser beam optical path, the top surface501 of the thin plate 500 provided on the top surface of the lens holder75 is positioned between the pair of sensor sections 511, 512 made of ametal plate of the capacitance-type minute displacement sensor 510provided outside the lens holder 75. The top surface 501 of the thinplate 500 and the pair of sensor sections 511 and 512 of thecapacitance-type minute displacement sensor 510 are designed to keep outof contact with each other and maintain a spacing that assures thespecific change of capacitance as a result of the focusing servo andtracking servo being applied to the lens holder 75, even when the lensholder 75 moves up and down and rotates minutely. When the top surface501 of the thin plate 500 is forced to intervene between the pair ofsensor sections 511 and 512 of the capacitance-type minute displacementsensor 510, this is the same as the state where a material with a highpermittivity is inserted between two metal plates, so if the pair as awhole is regarded as a capacitor, its capacitance increases. The changein capacitance is sensed by the minute displacement sensor 510, whichoutputs a sense signal. As a result, the small-aperture objective 35, orCD objective 35, is placed in a specific position and a sense signalindicating that the small-aperture objective 35 has been selected and isin the effective state is supplied to the objective identifying circuit104. When the large-aperture objective 34 is placed in the laser beamoptical path instead of the small-aperture objective 35, the top surface501 of the thin plate 500 is separated sufficiently from the pair ofsensor sections 511 and 512 of the capacitance-type minute displacementsensor 510 and the minute displacement sensor 510 is brought into thestate where the sensor 510 senses small capacitance, preventing thesense current from being sensed. As a result, the sense signalindicating that the small-aperture objective 35 is effective is notoutputted and the fact that the relatively large-aperture objective 34for DVD is effective is recognized by the objective identifying circuit104. Here, when the minute displacement sensor 510 is connected to ahigh-frequency generator circuit, a change in capacitance between thepair of sensor sections 511 and 512 will be sensed in the form of achange in resonant frequency, making it easy to sense the presence orabsence of the thin plate 500.

The capacitance-type minute displacement sensor 510 is a sensor thatsenses a change in capacitance and is widely used to determine thedisplacement of an object or the presence or absence of an object. Thissensor is based on the principle of a capacitor that when two metalplates with specific areas are forced to face each other in parallelwith a specific distance between them and electrodes are connected toboth of the metal plates, the amount of charge accumulated between thetwo metal plates varies with the permittivity of a substance inserted inthe spacing.

Such a sensor is of the non-contact type, so that it has no effect onthe operating characteristic of the lens holder 75 while the focusingservo and tracking servo are acting on the lens holder. Additionally, inthe present embodiment, neither the position nor displacement of theobject to be sensed need be measured with high accuracy, as long as onlythe presence or absence of the object to be sensed is sensed. Therefore,use of the minute displacement sensor 510 whose unit price is low andhas a limited function will accomplish the purpose sufficiently.Therefore, even when the minute displacement sensor 510 is built in theabove-described sensing mechanism, a highly reliable, highly stable lensidentifying mechanism can be realized without raising the cost of theapparatus.

While in the above embodiment, the sensing mechanism incorporating thecapacitance sensing minute displacement sensor 510 has been explained,the present invention is not limited to the capacitance sensing minutedisplacement sensor 510. The invention may be applied to another sensingmechanism, as long as the sensing mechanism is an element that sensesthe rotation of the lens holder under the principle of sensing a changein capacitance.

In FIG. 23, a sensing mechanism making use of supersonic waves is usedas the sensing mechanism that senses the lens position. As shown in FIG.23, on the top surface of the lens holder 75, a thin plate 600 isprovided. When one of the objectives 34 and 35 has been selected and theselected one is placed in the optical path, the tip 601 of the thinplate 600 is forced to face the tip 611 of a supersonic minutedisplacement sensor 610 provided outside the lens holder. When the otherobjective has been selected, the top surface 601 of the thin plate 600is moved to a place separate from the displacement sensor 610.

In the embodiment of FIG. 23, the thin plate 600 is positioned on adiagonal extending from the objective 35 with an almost small apertureto the rotational center of the lens holder 75. Therefore, when the lensholder 75 is rotated and the objective 35 is placed in the laser beamoptical path, the top surface 601 of the thin plate 600 provided on thetop surface of the lens holder 75 is forced to face the tip 411 of thesupersonic minute displacement sensor 610 located outside the lensholder 75. The top surface 601 of the thin plate 600 and the tip 611 ofthe supersonic minute displacement sensor 610 are designed to keep outof contact with each other and maintain a spacing that assures thesensing of the change of supersonic waves as a result of the focusingservo and tracking servo being applied to the lens holder 75, even whenthe lens holder 75 moves up and down and rotates minutely. When the topsurface 601 of the thin plate 600 is forced to face the tip 611 of thesupersonic minute displacement sensor 610, the supersonic wave generatedby the oscillating element of the minute displacement sensor 610 isreflected by the top surface 601 of the thin plate 600 and is sensed bythe receiving element of the minute displacement sensor 610, whichproduces a sense signal. As a result, the small-aperture objective 35,or CD objective 35, is placed in a specific position and a sense signalindicating that the small-aperture objective 35 has been selected and isin the effective state is supplied to the objective identifying circuit104. When the large-aperture objective 34 is placed in the laser beamoptical path instead of the small-aperture objective 35, the top surface601 of the thin plate 600 is separated sufficiently from the minutedisplacement sensor 610 and the supersonic wave emitted from theoscillating element is reflected by an optical unit 90 and is sensed bythe receiving element of the minute displacement sensor 610. In thiscase, it takes more time for the supersonic wave to return to thereceiving element of the minute displacement sensor 610 than when thetop surface 601 of the thin plate 600 is forced to face the tip 611 ofthe supersonic minute displacement sensor 610, which prevents the sensecurrent from being sensed. As a result, the sense signal indicating thatthe small-aperture objective 35 is effective is not outputted and thefact that the relatively large-aperture objective 34 for DVD iseffective is recognized by the objective identifying circuit 104. Thesupersonic minute displacement sensor 610 measures the time required forthe supersonic wave to return to the receiving element and memorizes themeasured time. When the sensor is connected to a comparator circuit, itcan produce a sense signal as described earlier.

The supersonic minute displacement sensor 610 determines thedisplacement of an object and the presence or absence of an object,making use of the properties of supersonic waves. Since supersonic waveshave high frequencies, they are characterized in that they have highdirectivity and almost all of their energy reaches the target. Onemethod of generating supersonic waves is an LC oscillation method inwhich direct current is supplied to a coil and a capacitor, for example.In addition, one method of receiving supersonic waves is a method ofreceiving supersonic waves with a piezoelectric element and directingthe voltage signal from the piezoelectric element to an LC oscillatingcircuit, which then converts the voltage signal into an electric signal.Such a sensor is of the non-contact type, so that it has no effect onthe operating characteristic of the lens holder 75 while the focusingservo and tracking servo are acting on the lens holder 75. Additionally,in the present embodiment, neither the position nor displacement of theobject to be sensed need be measured with high accuracy, as long as onlythe presence or absence of the object to be sensed is sensed. Therefore,use of the minute displacement sensor 610 whose unit price is low andhas a limited function will accomplish the purpose sufficiently.Therefore, even when the minute displacement sensor 610 is built in theabove-described sensing mechanism, a highly reliable, highly stable lensidentifying mechanism can be realized without raising the cost of theapparatus.

While in the above embodiment, the sensing mechanism incorporating thesupersonic minute displacement sensor 610 has been explained, thepresent invention is not limited to the supersonic minute displacementsensor 610. The invention may be applied to another sensing mechanism,as long as the sensing mechanism is an element that senses the rotationof the lens holder under the principle making use of supersonic waves.For instance, the invention may be applied to a sensing mechanism wherethe supersonic oscillating section is placed so as to face thesupersonic wave receiving section and a shielding plate is put betweenthese sections according to the position of the lens.

As described above, with the sensing means shown in FIGS. 19 to 23, theposition of the lens can be sensed reliably, the effective aperture ofthe objective placed in the laser optical path can be identified easilyon the basis of the sensing result of the lens position, and the opticalpickup can be controlled in a suitable mode according to theidentification result.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. An apparatus for searching a recording mediumusing a light beam, said apparatus comprising:a first objective lens anda second objective lens each having an optical axis; a light beamgenerator for generating a light beam directed along an optical pathextending between the light beam generator and the recording medium; alens holder having a rotational axis, said lens holder being constructedand arranged to support said first and second objective lenses; areflecting element associated with said lens holder, said reflectingelement being positioned and configured to reflect part of the lightbeam generated by said light beam generator and directed along theoptical path extending between the light beam generator and therecording medium; said lens holder being movable axially along therotational axis thereof to thereby move one of the first and secondobjective lenses axially along its respective optical axis; an objectivelens driver constructed and arranged to rotate said lens holder tothereby select one of said first and second objective lenses by placingthe selected objective lens in the optical path of the light beamgenerated by the light beam generator so that the selected objectivelens focuses the light beam on the recording medium, said objective lensdriver being constructed and arranged to move said lens holder axiallyalong the optical axis of the selected objective lens to thereby adjustthe focusing of the light beam by the selected objective lens and rotatesaid lens holder to thereby cause the selected objective lens to track adesired area of the recording medium; and an objective lens identifierconstructed and arranged to sense the part of the light beam reflectedby said reflecting element and identify which one of the first andsecond objective lenses has been selected and placed in the optical pathof said light beam.
 2. An apparatus according to claim 1, wherein saidobjective lens driver includes:first and second coils; and first andsecond magnets, said magnets and coils being positioned and configuredsuch that (1) energizing said first coil rotates said lens holder suchthat said first objective lens is selected and placed in the aforesaidoptical path of the light beam generated by said light beam generator,(2) energizing said second coil when said first objective lens isselected moves said lens holder axially along its rotational axis tothereby adjust the focusing of the light beam as it passes through saidfirst objective lens to the recording medium; (3) energizing said secondcoil rotates said lens holder such that said second objective lens isselected and placed in the aforesaid optical path of the light beamgenerated by said light beam generator, (4) energizing said second coilwhen said second objective lens is selected moves said lens holder alongits rotational axis to thereby adjust the focusing of the light beam asit passes through said second objective lens to the recording medium. 3.An apparatus according to claim 2, wherein said first magnet, firstcoil, second magnet, and second coil are arranged symmetrically withrespect to the rotational axis of said lens holder.
 4. An apparatusaccording to claim 2, wherein said objective lens identifier includes asensor for sensing part of the reflected light beam and generating asense signal, and an identification signal generator for generating anobjective lens identification signal as a result of the sense signal. 5.An apparatus according to claim 4, wherein said objective lensidentifier includes a switching circuit that, in response to saididentification signal, switches at least one of said coils from acontrol circuit that controls a tracking operation to a control circuitthat controls a focusing operation and also switches the same number ofcoils from the control circuit that controls a focusing operating to thecontrol circuit that controls a tracking operation.
 6. An apparatus forsearching a recording medium having a track with a light beam, saidapparatus comprising:first and second objective lenses each having anoptical axis, said optical axes being substantially parallel to oneanother; a light beam generator for generating a light beam whichtravels along an optical path between the light beam generator and therecording medium; a lens holder having a rotational axis substantiallyparallel with the optical axes of said first and second objectivelenses, said lens holder being constructed and arranged to support saidfirst and second objective lenses, said lens holder being rotatableabout and movable axially along the rotational axis thereof; a modesensing device positioned and configured to sense when said firstobjective lens is located in the optical path of said light beam andwhen said second objective lens is located in the optical path of saidlight beam, said mode sensing device including a reflecting elementassociated with said lens holder for reflecting a part of the light beamand a sensor for sensing the reflected part of the light beam andgenerating a sensing signal indicating whether said first objective lensor said second objective lens is located in the optical path of saidlight beam; first and second coils fixed to the lens holder and arrangedaround said rotational axis thereof; first and second magnets facing thefirst and second coils respectively, said first and second magnets beingspaced apart from said first and second coils; a switching signalgenerator for generating a switching signal to energize one of the firstand second coils, the lens holder being rotatable to move said firstobjective lens into the optical path of said light beam when theswitching signal is supplied to the first coils and to move the secondobjective lens into the optical path of said light beam when theswitching signal is supplied to the second coils; a signal generatorconstructed and arranged to generate a focusing signal and a trackingsignal in response to the light beam reflected from the recordingmedium; the focusing signal being supplied to the second coil when thefirst objective lens is located in the optical path and to the firstcoil when the second objective lens is located in the optical path tothereby move the lens holder along its rotational axis such that anassociated one of the first and second objective lenses placed in theoptical path of the light beam focuses the light beam on the recordingmedium; and the tracking signal being supplied to the first coil whenthe first objective lens is located in the optical path and to thesecond coil when the second objective lens is located in the opticalpath to thereby rotate the lens holder about its rotational axis suchthat the associated one of the first and second objective lenses placedin the optical path of the light beam guides the light beam to thetrack.
 7. An apparatus according to claim 6, wherein the first andsecond objective lenses have different optical characteristics forfocusing a light beam on the optical recording medium.
 8. An apparatusaccording to claim 6, wherein said first magnet, first coil, secondmagnet, and second coil are arranged symmetrically with respect to therotational axis of said lens holder.